1. Field of the Invention
The present invention relates to photolithographic techniques utilized in manufacturing semiconductor integrated circuits and, in particular, to a post exposure bake (PEB) technique for suppressing acid diffusion induced critical dimension changes in a chemically amplified photoresist process by reducing reaction activation energy to thereby reduce reaction time. Reaction activation energy is reduced by delivering energy required to overcome the reaction activation energy barrier directly into the chemical bonds involved in the reaction.
2. Discussion of the Related Art
Chemically amplified photoresists have been widely used in Deep UltraViolet (DUV) photolithography to manufacture integrated circuit devices with feature sizes smaller than 0.35 .mu.m. With DUV photoresists, strong acids are generated upon DUV illumination, as shown in FIG. 1A. In the subsequent post exposure bake (PEB) step, shown in FIG. 1B, the acids act as a catalyst, inducing chain-like chemical reactions that can change the dissolution properties of the resist. This process can result in a change A in the critical dimension, as shown in FIG. 1C, due to the isotropic characteristics of the acid diffusion.
The working principle of chemically amplified photoresists can be represented by the following equations (1) and (2):
At DUV exposure step, EQU Ph.sub.3 S.sup.+ X.sup.- +hv.fwdarw.X.sup.31 H.sup.+ +others (1)
At PEB step, ##STR1##
The diffusion of catalytic acids in chemically amplified resists has a strong influence on resist lithographic performance. On one hand, acid diffusion is necessary within the DUV exposed region in order to make chemical reactions occur. On the other hand, lateral diffusion of the acid, which can cause critical dimension change, is undesirable.
Ideally, the acid catalyzed chemical reactions should be completely acid diffusion controlled. That is, since the chemical reaction kinetics is much faster than the acid diffusion process, the overall chemical reaction rate may be determined by acid diffusion. In such a case, the time required to induce a certain amount of chemical reaction is minimized. Therefore, the lateral diffusion of the acid is also minimized.
In reality, the chemical reaction described by equation (2) above has a considerable "activation energy" barrier because not every collision between acid (H.sup.+) and the protected group (--COOC (CH.sub.3).sub.3) results in chemical reaction, even at elevated PEB temperature (.about.130.degree. C.). In fact, the probability of a successful collision that leads to chemical reaction is on the order of 10.sup.-12, if the activation energy is assumed to be 100 kJ/mol. This low successful collision probability results in a longer time being required to complete a certain amount of chemical reaction. Therefore the time available for acid (H.sup.+) diffusion is also extended, causing a larger change in the critical dimension.
Accordingly, there is a need for a technique for suppressing the impact of acid diffusion on critical dimension change in a chemically amplified photoresist process.